[coldtea]

>I find it slightly incredible that two planes ever hit each other given the size of planes compared to the volume of space they fly in.

The space can be constrained though. There are "airways" (jet routes) for flights, and not that much of a volume/size ratio when the volume concerns areas near airports -- where many planes approach, exit, are asked to circle in a holding pattern, etc.

So it's not like the whole sky is their domain...